This invention relates to friction stir heating and welding, and more especially to particular surface contours of the pin tool for generating heat.
Stir friction heating or welding is a relatively new technique, which involves plunging a moving nonconsumable tool into solid material to be heated or welded, as described in U.S. Pat. No. 5,460,17, issued Oct. 24, 1995 in the name of Thomas et al. Friction stir welding has shown great promise for the welding of metals which are otherwise difficult to weld. In friction stir welding, the moving tool plasticizes portions of the workpiece immediately adjacent to the moving tool. An advantage of the stir-friction heating or welding technique is that the material which is heated is not exposed to combustion products, which tends to avoid chemical changes of the workpiece attributable to interactions with the fuel and its combustion products. Another advantage is that the heated portions of the material tend to coalesce and solidify immediately upon removal of the tool. Also, the temperature of the workpiece, even in the heated region, tends not to be so high as that which may occur during conventional welding, which in turn tends to reduce oxidation of the workpiece due to ambient atmosphere. This reduced tendency toward oxidation, in turn, reduces the need for an inert atmosphere surrounding the region being heated or welded. In some materials, such as aluminum-lithium alloys, conventional welding also causes undesirable grain growth of the crystal structure that lowers the strength of the material.
In the particular case of butt-welding of metal plates along juxtaposed sides to form a seam, the moving tool advantageously takes the form of a rotating xe2x80x9cpinxe2x80x9d tool extending into the region between the plates. The pin tool ordinarily used for welding is made from a material having a higher melting temperature than the material of the workpiece, and should be harder than the workpiece material in order to prevent pin deformation during the initial phases of plunging into the workpiece. The ordinary pin tool takes the form of a generally cylindrical body defining a proximal end and a distal end. The proximal end of the pin tool is shaped to be attached to a rotating spindle, and the distal end is rounded. The plates of the workpiece are desirably restrained from moving away from each other and from the moving pin tool as described in the Thomas et al patent, but are not necessarily pressed together. Relative motion between the rotating tool and the joint to be welded into a seam is provided, as by moving the workpieces so that the rotating pin tool follows or tracks the joint.
The weld is started by controlling the spindle relative to the workpiece(s) so that the distal end of the pin tool is pressed against the two halves of the joint and rotated. The rotation produces friction, which plasticizes the workpiece material immediately adjacent the pin tool, thereby allowing the tool to plunge further into the joint region. When the proper plunge depth has been reached, the pressure tending to push the pin tool into the joint is reduced or eliminated, while the rotation of the tool continues. The plunging force is often reduced by the presence of a shoulder on the pin tool which contacts the upper surface of the workpiece when the plunge has reached the proper depth. U.S. Pat. No. 5,971,247, issued Oct. 26, 1999 in the name of Gentry, describes a technique for using rollers associated with the spindle and bearing on the workpiece to reduce the plunging effect at the proper depth. Once the appropriate plunge depth is reached, however controlled, the workpiece is translated relative to the rotating pin tool, so as to progressively apply the rotating tool to the remaining portions of the region to be welded. The shoulder on the pin tool serves the additional purpose of polishing the welded region as the pin tool leaves a just-formed welded joint.
It has been discovered that production of a high-quality welded joint requires that the pin tool penetrate to a precise depth. U.S. Pat. No. 6,168,066 B1, issued Jan. 2, 2001 in the name of Arbegast, describes a controller for controlling the plunge depth of a pin tool in response to the electrical characteristics of the weld. Other control techniques can be used to control the penetration depth.
U.S. Pat. No. 6,053,391, issued Apr. 25, 2000 in the name of Heideman et al. describes a problem with the stir friction welding of lap joints, in which the melt is stirred from top to bottom by a pin tool surface profile consistent with the threads of a bolt. The problem is that the material of the welded joint is deformed in a direction which depends upon the direction of rotation of the pin tool. The problem is solved by a radially projecting boss(es) defined by counter-wound grooves, which tend to stir the melt in both upward and downward directions, and by a recessed shoulder region.
Improved stir friction welding devices and techniques are desired.
A method for stir friction welding according to an aspect of the invention includes the step of procuring a stir friction pin tool having the general form of an elongated cylindrical body defining a distal end. The pin tool has a predetermined surface roughness about the cylindrical body, and a greater surface roughness at the distal end. According to the method, the pin is rotated about the axis of its cylindrical body, while plunging the distal end of the pin tool into a workpiece for friction welding thereof. The pin tool is translated transversely along the workpiece without significant further plunging to continue the stir friction welding of the workpiece. In one mode of the method, at least a portion of the tip of the distal end of the pin tool has a surface roughness of at least 128 microinches.
According to another aspect of the invention, the tip region of the pin tool has a surface roughness equal to or greater than 128 RMS or the equivalent. The surface roughness can be achieved by methods including the slotting of the tip, or drilling holes into the tip, or possibly by welding or sputtering an appropriate material onto the tip.
According to another aspect of the invention, a stir friction welding apparatus includes a stir friction pin tool defining a cylindrical body, a distal end and a proximal end. The pin tool has a given surface roughness about the periphery of the cylindrical body, some means for attachment adjacent the proximal end, and a surface roughness on the distal end which is greater than the given surface roughness. The welding apparatus further includes a spindle defining an axis and adapted for rotation thereabout. The spindle is also adapted for relative axial and transverse motion relative to a workpiece, and includes an attachment means adapted for attachment to the means for attachment of the pin tool. In operation, the pin tool is mounted to the spindle for rotation and transverse motion therewith. A drive arrangement is coupled to the spindle for rotating the spindle about the axis. A control arrangement is coupled to the spindle, for controllably causing the spindle to plunge the pin tool into a workpiece, and to translate relative to the workpiece.